Promoted silver acatalyst preparation

ABSTRACT

A silver catalyst for ethylene oxidation to ethylene oxide is prepared by impregnating an inert support with a silver/amine solution as well as with various promoters and calcining the impregnated support at 300°-500° C., the catalyst being maintained under an inert atmosphere at temperatures of 250° C. or higher, preferably at 100° C. or higher.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.08/348,340 filed Dec. 2, 1994, now U.S. Pat. No. 5,504,052.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the preparation of a supported silvercatalyst, which comprises various promoters and which is useful for thevapor phase oxidation of ethylene to ethylene oxide, by a processwhereby a support is impregnated with a silver salt/amine solution aswell as with various promoters and the resultant impregnated support iscalcined under controlled conditions of temperature and inert atmosphereto produce the silver catalyst.

2. Description of the Prior Art

Methods are known for the preparation of supported silver catalystsuseful for the vapor phase oxidation of ethylene to ethylene oxide,which methods involve impregnating a support such as alumina with asilver salt/amine solution. U.S. Pat. No. 3,702,359 is illustrative ofsuch procedures. The preparation of silver catalysts which also containalkali metal promoters by analogous procedures is shown, for example, inU.S. Pat. No. 3,962,136. Still further, similar procedures for thepreparation of silver catalysts promoted alkali metal and rhenium andalso with a co-promoter selected from sulfur, molybdenum, tungsten,chromium and mixtures are shown in U.S. Pat. No. 4,766,105.

Catalyst preparation by these prior art procedures has involvedimpregnating a support with the silver/amine solution which may containthe various promoters, and thereafter heating the impregnated support ina forced air oven up to a temperature of about 275° C. in order toreduce the silver to metallic silver and to separate volatiles from thecatalyst.

U.S. Pat. No. 5,444,034 relates to silver catalyst preparation wherein asupport is impregnated with a hydrocarbon solution of a silver salt ofan organic acid such as neodecanoic acid, activation in stages up to atemperature of 500° C. is shown under an inert gas such as nitrogen.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the present invention an improved method is providedfor the preparation of silver catalysts useful for the vapor phaseoxidation of ethylene to ethylene oxide. A conventional support such asalumina is impregnated with a silver/amine impregnating solution. Thesupport is also impregnated at the same time or in a separate step withvarious catalyst promoters. Subsequently, the impregnated support iscalcined at a temperature in the range of about 300° C.-500° C. for atime sufficient to reduce the silver component to metallic silver and toremove volatile decomposition products from the silver containingsupport. A critical feature of the instant catalyst preparationprocedure is that contact of the silver-containing support with anoxygen containing atmosphere is avoided at least at temperatures aboveabout 250° C., and preferably at temperatures in excess of 100° C. Bothduring the period when the impregnated support is heated to andmaintained at 300°-500° C., and during cooling of the calcined catalystfrom 300°-500° C. to 200° C. or lower, preferably 100° C. or lower, aninert atmosphere such as nitrogen or helium is maintained in contactwith the silver containing support.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron micrograph of a silver catalyst of theinvention calcined at 400° C. under nitrogen.

FIG. 2 is a scanning electron micrograph of a silver catalyst similarlyprepared but calcined at 270° C. under air in accordance with prior artprocedures.

FIG. 3 is a scanning electron micrograph of a silver catalyst similarlyprepared but calcined under air at 400° C.

FIG. 4 is a scanning electron micrograph of a silver catalyst of theinvention calcined at 400° C. under nitrogen.

FIG. 5 is a scanning electron micrograph of a silver catalyst preparedin a similar way to that of FIG. 4 but calcined at 270° C. under air.

FIG. 6 is a scanning electron micrograph of a silver catalyst preparedin a similar way to that of FIG. 4 but calcined at 400° C. under air.

FIG. 7 is a scanning electron micrograph of a fresh silver catalyst ofthe invention calcined at 400° under nitrogen.

FIG. 8 is a plot of selectivity versus time for the catalyst of FIG. 7.

FIG. 9 is a scanning electron micrograph of the silver catalyst of FIG.7 after several hundred hours of use.

FIG. 10 is a scanning electron micrograph of a silver catalyst similarlyprepared but calcined under air at 500° C.

FIG. 11 is a plot of selectivity versus time for the catalyst of FIG. 9.

FIG. 12 is a scanning electron micrograph of the silver catalyst of FIG.9 after several hundred hours of use.

DETAILED DESCRIPTION

Preferred catalysts prepared in accordance with this invention containup to about 30% by weight of silver, expressed as metal, deposited uponthe surface and throughout the pores of a porous refractory support.Silver contents higher than 20% by weight of total catalyst areeffective, but result in catalysts which are unnecessarily expensive.Silver contents, expressed as metal, of about 5-20% based on weight oftotal catalyst are preferred, while silver contents of 8-15% areespecially preferred.

In addition to silver, the catalyst also contains an alkali metalpromoter selected from potassium, rubidium and/or cesium, cesium beingpreferred. The promoter amounts and impregnation procedures described inU.S. Pat. No. 3,962,136 are advantageously employed.

Most preferably, the catalyst also contains a Group VIa promoter, ie atungsten, molybdenum and/or chromium promoter, illustratively in theform of an oxyanion such as tungstate, molybdate or chromate.Appropriate amounts and impregnation procedures are described in U.S.Pat. Nos. 4,9087,343, 5,057,481, 5,187,140, and 5,102,848, as well as5,011,807 and 5,099,041, and 5,407,888.

The catalyst can contain a rhenium promoter as described in U.S. Pat.No. 4,761,394 optionally with co-promoters as described in U.S. Pat. No.4,766,105.

The disclosures of the above patents are incorporated herein byreference as showing the various promoters, their amounts and method ofincorporation in the catalyst.

Especially preferred in accordance with the invention are silvercatalysts promoted by cesium and tungsten and/or molybdenum, optionallyfurther promoted with sulfur and/or fluorine.

Catalysts may be made with supports comprising alumina, silica,silica-alumina or combinations thereof. Preferred supports are thosecontaining principally alpha-alumina, particularly those containing upto about 15 wt % silica. Especially preferred supports have a porosityof about 0.1-1.0 cc/g and preferably about 0.2-0.7 cc/g. Preferredsupports also have a relatively low surface area, i.e. about 0.2-2.0 m²/g, preferably 0.4-1.6m² /g and most preferably 0.5-1.3 m² /g asdetermined by the BET method. See J.A. Chem. Soc. 60, 3098-16 (1938).Porosities are determined by the mercury porosimeter method; see Drakeand Ritter, "Ind. Eng. Chem. Anal. Ed.," 17, 787 (1945). Pore and porediameter distributions are determined from the surface area and apparentporosity measurements.

For use in commercial ethylene oxide production applications, thesupports are desirably formed into regularly shaped pellets, spheres,rings, etc. Desirably, the support particles may have "equivalentdiameters" in the range from 3-10 mm and preferably in the range of 4-8mm, which are usually compatible with the internal diameter of the tubesin which the catalyst is placed. "Equivalent diameter" is the diameterof a sphere having the same external surface (i.e. neglecting surfacewithin the pores of the particle) to volume ration as the supportparticles being employed.

As an essential feature of the invention, the silver is added to thesupport by immersion of the support into a silver/amine impregnatingsolution or by the incipient wetness technique. The silver containingliquid penetrates by absorption, capillary action and/or vacuum into thepores of the support. A single impregnation or a series ofimpregnations, with or without intermediate drying, may be used,depending in part upon the concentration of the silver salt in thesolution. To obtain catalyst having silver contents within the preferredrange, suitable impregnating solutions will generally contain from 5-50wt % silver, expressed as metal. The exact concentrations employed, ofcourse, will depend upon, among other factors, the desired silvercontent, the nature of the support, the viscosity of the liquid, andsolubility of the silver compound.

Impregnation of the selected support is achieved in a conventionalmanner. The support material is placed in the silver solution until allof the solution is absorbed by the support. Preferably the quantity ofthe silver solution used to impregnate the porous support is no morethan is necessary to fill the pore volume of the porous support.

The impregnating solution, as already indicated, is characterized as asilver/amine solution, preferably such as is fully described in U.S.Pat. No. 3,702,259 the disclosure of which is incorporated herein byreference. Alkali metal promoters, most preferably cesium, and theimpregnation procedures described in U.S. Pat. No. 3,962,136 areadvantageously employed as are the impregnation procedures described inU.S. Pat. Nos. 4,761,394, 4,766,105m, 4,908,343, 5,057,481, 5,187,140,5,102,848, 5,011,807, 5,099,041 and 5,407,888, the disclosures of all ofwhich are hereby incorporated by reference.

Known prior procedures of predeposition, co-deposition andpostdeposition of various promoters can be employed.

After impregnation, any excess impregnating solution is separated andthe support impregnated with silver and the promoter or promoters iscalcined or activated. The calcination is accomplished by heating theimpregnated support, preferably at a gradual rate, to a temperature inthe range 300°-500° C. for a time sufficient to convert the containedsilver to silver metal and to decompose the organic materials and removethe same as volatiles.

It is essential in accordance with this invention that the impregnatedsupport be maintained under an inert atmosphere while it is above 250°C. during the entire procedure. While not wishing to be bound by theory,it is believed that at temperatures of 250° C. and higher oxygen isabsorbed in substantial quantities into the bulk of the silver where ithas an adverse effect on the catalyst characteristics. Inert atmospheresas employed in the invention are those which are essentially free ofoxygen.

Catalysts prepared by the procedures above have improved performance,especially stability, for use in the production of ethylene oxide by thevapor phase oxidation of ethylene with molecular oxygen. These usuallyinvolve reaction temperatures of about 150° C. to 400° C., usually about200° C. to 300° C., and reaction pressures in the range of from 0.5 to35 bar. Reactant feed mixtures contain 0.5 to 20% ethylene and 3 to 15%oxygen, with the balance comprising comparatively inert materialsincluding such substances as nitrogen, carbon dioxide, methane, ethane,argon and the like. Only a portion of the ethylene usually is reactedper pass over the catalyst and after separation of the desired ethyleneoxide product and the removal of appropriate purge stream sand carbondioxide to prevent uncontrolled build up of inerts and/or by-products,unreacted materials are returned to the oxidation reactor.

The following examples illustrate the benefits of calcination of thepromoted catalysts in inert atmosphere according to the invention.

    ______________________________________                                        Silver Solution Preparation                                                   Chemical Compounds utilized (parts by weight):                                ______________________________________                                        Silver Oxide         688                                                      Oxalic Acid          369                                                      Ethylene diamine     409                                                      Deionized water      2340                                                     4% Cesium hydroxide in water                                                  ______________________________________                                    

Silver oxide (679 parts) was mixed with water, at room temperature,followed by the gradual addition of the oxalic acid. The mixture wasstirred for 15 minutes and at that point the color of the blacksuspension of silver oxide was changed to the gray/brown color of silveroxalate. The pH of the mixture was measured and was adjusted to a pHhigher than 7 via adding an additional amount of silver oxide. The totalamount of silver oxide added was 688 parts.

The suspension was allowed to settle and this was followed by decantingmost of the clear liquid that developed on top of the mixture. Thecontainer was placed in an ice bath and stirred while ethylene diaminewas added slowly to maintain the reaction temperature lower than 33° C.After the addition of all the ethylene diamine the solution was filteredat room temperature. The clear filtrate was utilized as a silver/aminestock solution for the catalyst preparation. In the following examples,parts are by weight unless otherwise specified.

EXAMPLE 1

Preparation Procedure

The support used for this preparation was obtained from Norton Companyand was made primarily of α-alumina in the form of 5/16" cylinders. Thesupport has a surface area of 0.55 m² /g, pore volume of 0.3 cc/g, andmedian pore diameter of 1.5μ. About 1110 parts of the silver solutionwas mixed with:

1. 11.88 parts of CsOH solution,

2. 13.9 parts of ammonium perrehanate, (3.77% Re in water), and

3. 1.8 parts of ammonium sulfate, (5% S in water).

The mixture was stirred to assure homogeneity, then added to 2500 partsof the support. The wet catalyst was mixed for ten minutes and thencalcined.

Calcination, the deposition of silver compound, was induced by heatingthe catalyst up to the decomposition temperature of the silver salt.This was achieved via heating in a furnace that has several heatingzones in a controlled atmosphere. The catalyst was loaded on a movingbelt that entered the furnace at ambient temperature. The temperaturewas gradually increased as the catalyst passed from one zone to the nextand was increased, up to 400° C., as the catalyst passed through sevenheating zones. After the heating zones the belt passed through a coolingzone that gradually cooled the catalyst to a temperature lower than 100°C. The total residence time in the furnace was 22 minutes. Theatmosphere of the furnace was controlled through flow of nitrogen to thedifferent heating zones. Nitrogen was passed upwardly through thecatalyst in each zone to aid in the removal of volatiles and to provideand atmosphere essentially free of oxygen.

FIG. 1 is an electron micrograph of this catalyst.

EXAMPLE 2 (Comparative)

Catalyst preparation was the same as Example 1 except that calcinationwas conducted in air and to a maximum temperature of 270° C. FIG. 2 isan electron micrograph of this catalyst.

EXAMPLE 3 (Comparative)

Catalyst preparation was identical to Example 1 except the calcinationwas conducted in air and to a maximum temperature of 400° C. FIG. 3 isan electron micrograph of this catalyst.

EXAMPLE 4

Preparation Procedure

The support used for this preparation was obtained from Norton Companyand was made primarily of α-alumina in the form of 5/16" cylinders. Thesupport has a surface area of 0.55 m² /g, pore volume of 0.3 cc/g, andmedian pore diameter of 1.5μ. about 1110 parts of the silver solutionwas mixed with 8.56 parts of CsOH solution. The mixture was stirred toassure homogeneity. then added to 2500 parts of the support. The wetcatalyst was mixed for ten minutes and then calcined.

Calcination, the deposition of silver compound, was induced by heatingthe catalyst up to the decomposition temperature of the silver salt.This was achieved via heating in a furnace having several heating zonesin a controlled atmosphere. The catalyst was loaded on a moving beltthat entered the furnace at ambient temperature. The temperature wasgradually increased as the catalyst passed from one zone to the next andwas increased, up to 400° C., as the catalyst passed through sevenheating zones. After the heating zones the belt passed through a coolingzone that gradually cooled the catalyst to a temperature lower than 100°C. The total residence time in the furnace was 22 minutes.

The atmosphere of the furnace was controlled by nitrogen flow to thedifferent heating zones. Nitrogen was passed upwardly through thecatalyst in each zone to aid in the removal of volatiles and to providean atmosphere essentially free of oxygen. FIG. 4 is an electronmicrograph of this catalyst.

EXAMPLE 5

Preparation Procedure

Catalyst preparation was identical to Example 4 except the calcinationwas induced in air and to a maximum temperature of 270° C. FIG. 5 is anelectron micrograph of this catalyst.

EXAMPLE 6 (Comparative)

Preparation Procedure

Catalyst preparation was identical to example 4 except the calcinationwas induced in air and to a maximum temperature of 400° C. FIG. 6 is anelectron micrograph of this catalyst.

The above catalysts were tested for activity and selectivity by crushingand placing 36 grams in a micro reactor consisting of a 1/4 " stainlesssteel tube which was heated in a salt bath. A feed mixture by volume of7% oxygen, 8% CO₂, 15% C₂ H₄, 70% N₂ was passed over the catalyst with agas space velocity of 5500 hr⁻¹. The pressure was maintained at 300 psig(21.69 bar) and the temperature between 200° C. and 300° C. as requiredto maintain an outlet concentration of 1.5 vol % (160 Kg per hour per m³of catalyst) ethylene oxide. The activity of the catalyst is expressedas the temperature necessary to maintain the outlet concentration of1.50 vol % ethylene oxide, the lower the temperature, the more activethe catalyst. The selectivity of the catalyst is expressed as the mol %of the total ethylene converted to ethylene oxide at the outletconcentration of 1.50 vol % ethylene. The stability of the catalyst ismeasured by the increase in temperature required to maintain theethylene oxide productivity.

The results shown in following Table clearly demonstrate the superiorityof the catalysts prepared in accordance with the invention, Examples 1and 4, as compared to analogous catalyst prepared by prior artprocedures.

                  TABLE 1                                                         ______________________________________                                                Promoters/                                                            Example Levels    Calcination                                                                              Activity                                                                             Results                                   #       ppm       Temp/atm.  °C.                                                                           Sel %                                     ______________________________________                                        1       Cs/Re/S = 400° C./N.sub.2                                                                   248    83.7                                              420/186/32                                                            2       Cs/Re/S = 270° C./air                                                                       256    LOW EO*                                           420/186/32                  (1.17)                                    3       Cs/Re/S = 400° C./air                                                                       259    LOW EO*                                           420/186/32                  (1.33)                                    4       Cs = 300  400° C./N.sub.2                                                                   228    81.5                                      5       Cs = 300  270° C./air                                                                       236    81.5                                      6       Cs = 300  400° C./air                                                                       238    81.7                                      ______________________________________                                         *The catalyst were quite inactive. At temperatures close to 260° C     the amount of ethylene oxide was lower than the desired 1.5%.            

EXAMPLE 7

A catalyst comprised of Ag/Cs/Mo/S/F was prepared by the procedure ofU.S. Pat. No. 5,102,848. The impregnating solution was as described inExample 1 and comprised a silver oxalate/ethylene diamine complex: withsilver in a concentration designed to give 12.5-14% by weight silver inthe final catalyst. The impregnating solution also contained cesiummolybdate, ammonium fluoride, cesium hydroxide and sulfuric acid andcomprised 2555 g silver/amine solution, 18 g Cs solution (8 wt % Cs inwater, charged as CsOH), 36 g Mo solution (4.26 wt % Cs₂ MoO₄ in water),16 gS solution (1 wt % S in water charged as H₂ SO₄) and 40 g ammoniumfluoride solution (1.5 wt % NH₄ F in water) to give the followingpromoter concentrations (expressed as the element) in the finalcatalyst:

                  TABLE 2                                                         ______________________________________                                        Promoter   Concentration, ppm by weight                                       ______________________________________                                        Mo          75                                                                Cs         500                                                                S           35                                                                F          140                                                                ______________________________________                                    

The support and preparation procedure of Example 1 was used.

The impregnated support was calcined in accordance with the inventionunder a nitrogen atmosphere using a furnace having several zones in acontrolled atmosphere. The catalyst was loaded on a moving belt thatentered the furnace at ambient temperature. The temperature wasgradually increased as the catalyst passed from one zone to the next andwas increased, up to 400° C., as the catalyst passed through sevenheating zones. After the heating zones the belt passed through a coolingzone that gradually cooled the catalyst to a temperature lower than 100°C. The total residence time in the furnace was 20 minutes. Theatmosphere of the furnace was controlled through flow of nitrogen to thedifferent heating zones. Nitrogen was passed upwardly through thecatalyst in each zone to aid in the removal of volatiles and to providean atmosphere essentially free of oxygen.

The catalyst was under an atmosphere of nitrogen from the time it wasloaded on the belt until it had been cooled to below 100° C.

FIG. 7 is an electron micrograph of the catalyst from which it can beseen that the catalytic components were evenly and finely dispersed onthe support surface.

The catalyst was tested for selectivity by crushing and placing 36 gramsin a micro reactor consisting of a 1/4" stainless steel tube which washeated in a salt bath. A feed mixture by volume of 7% oxygen, 8% CO₂,15% C₂ H₄, 70% N₂ was passed over the catalyst with a gas space velocityof 5500 hr-1. The pressure was maintained at 300 psig (21.69 bar) andthe temperature was adjusted as required to maintain an outletconcentration of 1.54 vol % (160 Kg per hour per m³ of catalyst)ethylene oxide. The selectivity of the catalyst is expressed as the mol% of the total ethylene converted to ethylene oxide at the outletcontraction of 1.50 vol % ethylene. FIG. 8 is a plot of selectivityversus time and demonstrates the high stability of the catalyst of theinvention.

FIG. 9 is a scanning electron micrograph of the catalyst after 210 hoursof use and when compared with FIG. 7 illustrates the high resistance tosintering of catalysts prepared according to the present invention.

EXAMPLE 8 (Comparative)

A catalyst was prepared as described in Example 7 except that thecatalyst was calcined in accordance with the teachings of U.S. Pat. No.5,102,848.

Specifically, the catalyst was calcined on a moving belt at 500° C. inan air atmosphere.

FIG. 10 is a scanning electron micrograph of the calcined catalyst andillustrates that the catalytic components were evenly and finelydispersed on the support surface.

The catalyst was tested for ethylene oxide production as described inExample 7. FIG. 11 is a plot of selectivity versus time for the catalystand clearly shows that although the catalyst initially is quiteselective, the selectivity declines rapidly with time.

FIG. 12 is a scanning electron micrograph of the catalyst after 190hours use and illustrates the high degree of sintering which occurs onthe surface of supports for catalysts prepared by prior art procedures.

We claim:
 1. In a process for the preparation of a supported silvercatalyst suitable for use in the oxidation of ethylene to ethylene oxidewherein an inert support is impregnated with a silver/amine solution andwith one or more alkali metal selected from potassium, rubidium andcesium and calcined, the improvement which comprises heating theimpregnated support to 300°-500° C. for a time sufficient to convert thesilver to metallic silver and to decompose and remove organic materials,the impregnated support being maintained under an inert gas atmosphereessentially free of oxygen at temperatures of 250° C. or higher.
 2. Theprocess of claim 1 wherein the impregnated support is maintained underan inert gas atmosphere essentially free of oxygen at temperatures of100° C. or higher.
 3. The process of claim 1 wherein the inert gas isnitrogen.
 4. The process of claim 1 wherein the alkali metal is cesium.5. The process of claim 1 wherein the support is impregnated with apromoter combination comprised of cesium and tungsten, molybdenum and/orchromium.
 6. The process of claim 5 wherein the promoter combinationalso comprises sulfur and/or fluorine.